The present invention relates to the plating of high purity aluminum or its alloys (hereinafter and in the claims, simply termed "aluminum") onto an electrically conductive substrate serving as the cathode in an electrolytic cell.
The electroplating of aluminum has been studied and investigated for over half a century by various workers. Prior art aluminum plating processes have been reviewed in detail in the applicant's parent application, Ser. No. 645,552, now U.S. Pat. No. 4,003,804, issued Jan. 18, 1977, and incorporated herein by this reference. In general, the majority of prior art processes are not feasible for large scale industrial application for a variety of reasons. Some processes are associated with known fire and toxicity hazards; some yield aluminum deposits that are not high purity; some electrolytic baths have normally been unstable; and some are complex and relatively expensive to formulate.
With regard to a recent publication by Peled and Gileadi entitled "The Electrodeposition of Aluminum from Aromatic Hydrocarbons", J. Electrochem. Soc., 123, 15 (1976) and a similar article entitled "Electroplating of Aluminum From Aromatic Hydrocarbons" in Plating 60, 342 (1975) it is stated that excellent aluminum deposits can be obtained from stable baths which employ relatively safe chemicals.
Peled and Gileadi describe a plating bath which requires the presence of 2-3 M aluminum halide and 0.1-1 M alkali halide, an organic non-Lewis base solvent (such as benzene and toluene) and a Lewis base such as ethylbenzene. (A Lewis base is any substance that will donate an electron pair to an electron pair acceptor (Lewis Acid)). Ordinarily, benzene and toluene are not considered to have any Lewis base characteristics for practical purposes; however, as a theoretical matter they do have weak Lewis base characteristics. For purposes of this specification and claims, toluene and benzene are considered as non-Lewis base solvents.
The Peled and Gileadi process appears to have intrinsic drawbacks; thus:
Peled et al.'s plating baths consist of basically four (4) ingredients (while the present process requires three (3) basic ingredients). Thus, in addition to AlBr.sub.3, an organic solvent such as benzene or toluene, and an alkali bromide, the Peled et al., bath requires a relatively strong Lewis base such as ethylbenzene, a large aromatic hydrocarbon, or an amine in order to yield acceptable plating. The Lewis base is apparently thought to be required by these inventors to "render the bath insensitive to trace amounts of water or HBr". Also, Lewis bases are considered to be good "bromine getters", and their presence would prevent or retard any deleterious effects of halide or halogen formed during plating by absorbing the by-products of plating. In the present invention, a "bromine-getter" or "halogen-getter" is not required because: (a) neutral solvents inert to halogens such as benzene, tetrahydrofuran, and cyclohexane are used; and (b) the electrolytes of the present process, which contain the by-products of plating, are directed from the zone of plating to a column packed with activated aluminum which regenerates more aluminum cationic plating species and this regenerated electrolyte is then recirculated to the plating zone to replace the aluminum cationic plating species consumed during plating.
At any rate, Peled and Gileadi report that when a single solvent non-Lewis base system such as benzene or toluene, is employed in their plating baths, "a tarlike organic deposit containing no aluminum was formed if benzene was the solvent" and in toluene "a gray to black deposit was produced". They therefore concluded that the addition of a strong Lewis base was essential to a good plating performance. (In stark contrast, the applicant herein has obtained the best quality aluminum deposits, at highest current densities, when a single non-Lewis base solvent system, such as toluene or benzene, is used without the addition of a fourth component as in the case of the above inventors).
Some Lewis bases such as ethylbenzene and naphthalene derivatives are relatively expensive and in scarce supply. Furthermore, the employment of a Lewis base in the plating system substantially increases the resistivity of the bath. Because of such increased resistivity Peled and Gileadi report that in a majority of cases their current densities are only 5 to 10 mA/cm.sup.2 at 100% current efficiency. (In the present invention, substantially increased current densities-varying from 35 to 125 mA/cm.sup.2 are observed at 100%, or near 100% current efficiency.)
The work of Peled, and Gileadi, cited earlier, used preformed aluminum bromide, as do Capuano and Davenport*. Preformed aluminum bromide is relatively expensive, is in scarce supply, and is difficult to work with because it picks up water readily from the atmosphere. Water readily reacts with the preformed aluminum bromide to form aluminum oxide; thus aluminum is not available for the plating operation. On the other hand, the applicant herein prepares aluminum bromide in complex form in situ, which is relatively safe to employ on a large scale. Furthermore, other aluminum halides such as AlCl.sub.3 and AlI.sub.3 have also been employed either, by themselves or in mixtures thereof, in some baths of the present invention (see Table I). FNT * see Capuano and Davenport, Canadian Pat. No. 945,935 and U.S. Pat. No. 3,775,260
With further regard to the Mathers et al. work, exemplified by U.S. Pat. No. 2,170,375 issued Aug. 22, 1939 and The Electrochemical Society, Pre-Print 65-2, published Apr. 30, 1934, pp. 25-38, the Mathers electrolyte of AlBr.sub.3 -Ethyl Bromide-Benzene forms an ethyl aluminum bromide cation plating complex ([EtAl.sub.2 Br.sub.5 ].sup.+, which is formed spontaneously between AlBr.sub.3 and Ethyl Bromide (or other alkyl halides). The Peled and Gileadi developments, discussed above, also produces a similar plating complex between a Lewis base such as ethylbenzene and AlBr.sub.3. In both the Mathers and Gileadi electrolytes, low current densities result and the electrolytes are non-regeneratable. The solvents, such as benzene and toluene, do not form part of the plating complex.
The electrolytes formed by means of the present invention differ substantially in concept from those of Gileadi and Mathers in producing a different plating complex which incorporates the solvent itself. The electrolytes of this invention require, initially, the presence of hydrogen halide to form the particular plating complex. However, the presence of the hydrogen halide must be carefully restricted to low limits after the plating complexes have been formed in order to accomplish the overall plating results desired by the applicant.
The resulting electrolytes of this invention are not only regeneratable but permit higher current densities and greater throwing power than those of Gileadi and Mathers. In addition, cathode efficiency and plating quality are superior to these prior art electrolytes. Furthermore, compared to prior art processes known up to the present time, this invention provides the most economical and relatively-safe-to-employ method to electrodeposit high quality aluminum on a large scale. The electrolytes of the present invention are the most thermodynamically stable yet.